Agricultural byproducts, including pineapple peel (PP), are valuable feed additives which support the livestock industry. However, conflicting evidence exists regarding the optimal amount of PP required to achieve optimal fermentation in silage. This study examines the impact of ensiling mixtures of equal proportions of fresh Napier grass (NG) and sugarcane top (ST) with varying levels of PP (0% [C], 10% [P1], 20% [P2], and 30% [P3]) on fermentation quality, microbiological profiles, and in vitro ruminal digestion. Compared to the C silage, the dry matter, crude protein, and neutral detergent fiber contents decreased in the silage treated with increasing PP ( < 0.05). P1 exhibited lower ( > 0.05) pH, higher ( > 0.05) lactic acid content, and lower ( < 0.05) NH-N content than other silage. The Chao 1, ACE index, and relative abundance of and Lactobacillales were decreased following the order of C > P1 > P2 > P3 ( < 0.05). Although there were no significant differences observed in most vitro ruminal fermentation parameters among four silages ( > 0.05), P1 exhibited higher total gas production, total volatile fatty acid, acetate acid, acetate-to-propionate ratio, and lower pH than the other silages. These results demonstrated that a NG and ST mixture co-ensiling with appropriate PP enhances the NG and ST mixture silage quality, and the optimum addition ratio for PP was 10% on a fresh matter basis.